312 



POPULATIONS 



it is not figured. SufiBce it to say that 

 Davidson censused the thrips at ten day in- 

 tervals for approximately 100 days and fit- 

 ted these data to a logistic curve. The fit 

 is fairly close up to the point of inflection 

 of the curve, but becomes erratic as the 

 asymptote is approached. 



Figures 103 and 104 are logistic curves 

 fitted to the sheep population statistics of 

 South Australia and Tasmania by Davidson 

 (1938, 1938a), Since creditable records of 

 number of sheep have been kept by live- 

 stock agencies and since the records extend 

 back to 1840 for Australia and to 1819 for 

 Tasmania, it is possible to describe the 



follow the curve closely, and the dry years 

 appear to have had little effect in reducing the 

 population. This may be attributed to the ex- 

 tensive, unoccupied country available for graz- 

 ing during this early period. For this reason, 

 cattle, which attained their maximum numbers 

 of 375,000 in 1860 and rapidly fell away again, 

 do not appear to have entered seriously into 

 competition with sheep for pasture. The fall in 

 the population during 1869-72 is associated 

 with the dry years, 1868, 1869, and the 

 economic depression which obtained during 

 this period. The persistent fall in the population 

 during 1884-89 is related to the dry years of 

 the 1880's; under the influence of good rains 

 in 1889-90 the numbers again rose rapidly 

 [and the asymptote was attained]." 



20 



25 



10 15 



TIME IN DAYS 



Fig. 105. Population growth of the diatom, Nitschia closterium. The upper curve is total 

 population; the lower curve is number of sessile cells. ( From Riley. ) 



growth of these populations essentially from 

 their estabhshment to postasymptotic equi- 

 librium. Looking at Figure 103 for Aus- 

 tralia, it is immediately apparent that the 

 agreement between points and curves is 

 favorable. The data for Tasmania (Fig. 

 104) do not fit nearly so well; there are de- 

 viations of considerable magnitude near the 

 asymptote. 



Davidson comments on the curve for 

 South Australia as follows: 



"The population follows closely the trend of 

 the calculated curve throughout this period. 

 The sheep numbers for 1838 and 1839 are 

 dominated by importations and fall below the 

 calculated curve. From 1840 to 1868 they 



Although it seems probable that the 

 growth of practically all natural, aquatic 

 populations may be represented by the sig- 

 moid curve, it has been diflBcult to find 

 cases for which the equation has been ap- 

 plied to actual data. There are incidental 

 suggestions for this in R. H. Fleming 

 (1939) for marine zooplankton organisms 

 and in Graham (1935) for marine fishes of 

 commercial importance. This dearth is prob- 

 ably in part related to the fact that such 

 populations in nature are characteristically 

 in a growth form already beyond the period 

 of positive growth. As G. E. Hutchinson 

 points out,** an excellent source for such 



• Personal conununication. 



